Sintering of loose particulate aluminum metal



United States Patent ABSTRACT OF THE DISCLOSURE A rigid sinteredaluminum metal article is made by the method of heating a charge ofloose, particulate aluminum metal in a mold in a closed gas-imperviouscontainer to a temperature and for a time at which a predeterminedportion of the charge, in the range of -70 percent by weight, becomesmolten. The particulate metal is substantially free of oxide filmsthicker than 700 A. Formation of a rigid integral article issubstantially assured if the metal is protected from rapid oxidationduring at least two of the steps of making the particulate metal,storing the same and sintering charges thereof.

This invention relates to the sintering of loose particulate aluminummetal to form a rigid article. It is more particularly directed to thepreparation of highly porous aluminum metal articles, such as articlesuseful in the form of oil filters and air filters for internalcombustion engines, as well as more impervious solid articles havinghigher structural strength.

For the purposes of the specification and claims, aluminum andaluminum-base alloys containing at least 70 percent by weight ofaluminum are hereinafter referred to as aluminum metal.

Conventional preparation of sintered articles from powdered metals suchas has or bronze powder is carried out by first cold pressing the looseparticulate metal to form a compact and heating the compressedparticulate metal just sufliciently to obtain sintering withsubstantially no melting of the metal. However, the con ventionalmethods have been singularly unsuccessful when applied heretofore to thesintering of particulate aluminum.

It is therefore a principal object of the invention to pro vide a methodfor sintering loose particulate aluminum metal.

Another object of the invention is to provide a method of sinteringloose particulate aluminum metal which avoids the necessity of making acompact, as by a cold pressing operation, before sintering.

Another object of the invention is to provide a method of avoiding heavyoxide films in the preparation and sintering of particulate aluminummetal.

Yet another object of the invention is to provide a method of making asintered article of aluminum metal having a preselected degree ofporosity.

A further object of the invention is to provide a method of obtaining animproved, rigid, porous article of aluminum metal having desirably highstrength properties.

Another object of the invention is to provide a method of preparing arigid, coherent substantially fully densified article in the sinteringof particulate aluminum metal.

Still another object of the invention is to provide a method ofpreparing a sintered article of aluminum metal having a reproducible,preselected degree of porosity and yet uniformly good structuralstrength from article to article.

These and other objects and advantages of the present invention areobtained by providing substantially or suificiently oxide-free, loose,particulate aluminum metal prepared under properly controlledconditions, as hereinafter more fully described; placing a charge ofsuch particulate aluminum metal in a mold in a container which is closedto exclude oxidizing atmosphere; and heating the charge of looseparticulate metal in a dry, substantially inert atmosphere and to atemperature and for a time that at least about 15 percent by weight ofthe charge becomes molten, but complete melting of the charge does nottake place; and thereafter permitting the partially melted metal to cooland solidify, thereby forming a strong, rigid, aluminum metal article.

The presence of an oxide layer on the surface of particulate aluminummetal interferes with the sintering thereof by forming a barrier layerbetween the particles which is difiiculty bridged even by molten metalduring the sintering process. Although it is desirable to minimizeoxidation to insure optimum sinterability, appreciable oxidation can betolerated without destroying the sinterability of the aluminum pellets.The critical oxide layer thickness which must not be exceeded if goodsintering is to be obtained consistently appears to be about 700Angstroms. Since oxidation can occur during each of the steps ofpreparing, storing and sintering the particulate metal, it is imperativethat each step be controlled so that the final oxide thickness at themoment fusing and bonding is to occur is below the critical thickness.

Aluminum metals which may be desirably transformed into sinteredarticles by the present process include aluminum and any of thealuminum-base alloys containing at least 70 percent by Weight ofaluminum. Preferably, the alloy has a fairly wide solidification range,e.g., a range of at least 25 Fahrenheit degrees. Examples of aluminumalloys having a suitable melting range and desirably employed in thepresent process are the binary aluminum alloys containing, respectively,about 5 percent of copper (AL-SCu), about 2 percent of silicon (Al-ZSi)and about 8 percent of magnesium (Al-8Mg).

The foregoing aluminum alloys all exist as homogenous melts atconventional casting or atomization temperatures (1250 to 1400 F). Adifferent type of aluminum metal advantageously sintered according tothe invention is the aluminum alloy which does not exist as a homogenousmelt at conventional casting or atomization temperatures. Such alloysform separate phases, the molten phase, and a discrete solid phase.Since these alloys settle or stratify during treatment of the meltbefore casting, they are not castable as such and particulate metalcontaining the intended constituents cannot be prepared by this route.

To bring such an alloy to particulate form without loss of alloyingconstituents, it is therefore necessary to atomize the alloy at atemperature at which the alloy is homogenous These alloys arehereinafter referred to as normally heterogeneous alloys. The normallyheterogeneous alloys advantageously treated according to the inventionconsist of at least 20 percent by weight, and more preferably at least50 percent by weight of aluminum or homogenous aluminum alloy, and thebalance a component Which is insoluble at normal casting temperatures,or the balance may be an intermetallic compound with aluminum which isinsoluble in the melt at normal casting temperatures. Examples ofinsoluble components which may be used are silicon and beryllium.Examples of other suitable elements and the intermetallic compounds theyform are listed as follows:

9 Element: Intermetallic compound B AlB FE FeAl Mn MnAl Z1 ZrAl3 3Examples of specific suitable alloys of this type are: (l) 30 percent byweight of silicon, the balance aluminum, and (2) 20 percent by weight ofmanganese, and the balance aluminum.

Homogenous aluminum metal may be obtained in particulate form bymachining solidified massive aluminum metal, or, by disc or jetatomizing the molten metal, as well understood in the art, in a dry,preferably rn-0XidiZ ing atmosphere. Examples of a suitable atmosphereare dry natural gas, air having a dewpoint of F. or lower (i.e.,containing less than 0.3 percent by volume water vapor), or a vacuum.The machined chips or atomized pellets may be of any size or finenesspractical for making a useful sintered product. Atomized pellets areusually finer than 20 mesh when formed. Generally, the machined chipsshould not have a maximum dimension greater than about /2 inch. Disc orjet atomizing is the preferred method of preparing particulate aluminumfor the method of the present invention.

The alloys which are only moderately active, such as the aluminum-copperor aluminum-silicon alloys, may be prepared in particulate form eitherby machining the massive metal, i.e., by mechanical comminution, or, byone of the atomizing processes. It is highly desirable that aluminumalloys containing constituents such as magnesium or lithium are obtainedin particulate form by disc or jet atomizing the molten metal in a dry,substantially inert atmosphere.

As indicated above, atomizing is the preferred method of preparingparticulate homogeneous aluminum alloys, although mechanical comminutionmethods may be employed if oxidation is properly controlled. Al-SCu andAl-8Mg were found to sinter much more readily when the particulate metalwas obtained by atomizing molten metal rather than by machiningsolidified metal. Atomized pellets of both said alloys sintered welleven after exposure to the ambient atmosphere for several months at roomtemperature. Machined chips of Al-SCu which had been encapsulated inglass shortly after preparation sintered quite adequately. Machinedchips of the same alloy which were not encapsulated were found notsinterable after exposure to the ambient atmosphere for one week at roomtemperature. Chips formed of Al-8Mg and machined in air were found notto sinter at all regardless of storage conditions. Mechanicalcomminution methods are more acceptable when carried out in an inertatmosphere.

The atomizing atmosphere employed in disc atomizing or the stream of gasemployed in jet atomizing should be as inert as possible. Contaminationof the atmosphere or jet with oxygen is not per se serious, but thepresence of water vapor is to be avoided as much as reasonably possible.Aluminum pellets prepared in an atmosphere or jet of natural gas orargon, or even in air, can be readily sintered if the water vaporcontent of the gas in each case is kept quite low. In each case, toassure sinterability, the water vapor content should desirably by keptbelow about 0.15 percent by weight water. Collecting the pellets inwater during the atomizing process also tends to impair sinterability.Pellets obtained by atomization in air containing 0.44 percent waterwere sinterable when the pellets were collected dry, but were notsinterable when the pellets were collected in water.

A highly inert gas is used for atomizing when spherical pellets aredesired, while air is used for producing irregularly shaped particles.Because greater oxidation occurs when air is used as the atomizing gas,somewhat closer control is required during subsequent storage andsintering steps than would be required for pellets atomized in a highlyinert gas such as argon.

In atomizing the normally heterogeneous alloys, the temperature of themelt is maintained sufficiently high for the melt to be entirely moltenand homogenous during the atomizing operation. Temperatures of 1400 to2000 F. are employed, though still higher temperatures could be used ifcontainer materials resistant to the action of high temperature moltenaluminum were available. At temperatures above about 1600 F. thereactivity of the molten aluminum is greater so that it is desirable,when atomizing molten aluminum at temperatures above about 1600 F. totake greater care to avoid water vapor or other reactive gas in theatomizing gas and environment.

In each of the particle making, storing, compacting and sinteringoperations described herein, it is particularly important to avoidreaction of the aluminum with water vapor. The effect isnot completelyunderstood, but it may be due to the formation of a thicker bulkieroxide than is had on reaction with oxygen.

If the particulate metal is not used immediately after its preparationto make a sintered article, the metal is stored in a dry air environmentto minimize oxidation. The particulate metal may be stored moreprotectively in a dry inert gas atmosphere, if desired. Aluminum pelletscan be successfully sintered several years after being prepared byatomizing if they are properly stored. Storage in air is satisfactory iflow humidity is maintained. Al-2Si pellets atomized in natural gas havebeen found not sinterable if exposed at 100 F. for more than about sevendays in air saturated with water vapor. The sinterability of Al-2S ipellets atomized in natural gas is not adversely affected on exposure ofthe Al-2Si pellets to the atmosphere for 20 days if the storagetemperature does not exceed about 75 F., or, if most of the water vaporis removed from the air. If the Al-2Si pellets have been atomized inair, the criticalexposure time under conditions of 100 F. temperatureand about 100% relative humidity is reduced to less than one day.Pellets of Al-8Mg exposed to the atmosphere at 75 F. and 100% relativehumidity are not sinterable after four days exposure. Pellets of Al-SCuhave been successfully sintered after being held in storage in an airconditioned room for a month under conditions of about 40 to 60%relative humidity, but the same alloy pellets usually becomeunsinterable in about five to seven days on being exposed to relativehumidity or higher at room temperature as found in an open warehouse ona rainy day.

Desirably, the particulate metal is stored in a dry atmosphere ofnatural gas, nitrogen, helium or argon containing less than about 0.5percent by volume of oxygen and less than 0.3 percent by volume of watervapor, the atmosphere being substantially free of gases such as carbondioxide and chlorine. Metal in the form of atomized pellets may bestored in dry air containing less than about 0.3 percent by volume ofwater vapor. Generally, it is sufficient and practical to simply storethe pellets in a closed receptacle such as a fiberboard drum. The use ofa desiccating agent and an inner plastic liner for the receptacleprovides additional protection from abbient atmospheric moisture.

In carrying out the sintering process of the invention, looseparticulate aluminum metal is provided which has been prepared so as tobe substantially free of thick oxide films, as described above, andwhich, if not freshly made, has been properly stored. The looseparticulate metal is placed in a mold which is desirably sealed off andplaced in a furnace. If desired, the mold may be placed in a containerwhich is adapted to be filled and/ or flushed continuously with an inertgas during the sintering period. The container is placed in a furnace ofappropriate size and heating capacity. It is also possible to use aproperly closed furnace itself as the container and to flush theinterior with an inert gas and to maintain an inert atmosphere in thefurnace during the sintering step.

A critical aspect of the sintering step is the selection of the materialfor construction of the mold, since most materials give off oxidizinggases, e.g., H O vapor and 0 when heated, and since molten aluminum hasa strong tendency to alloy with or bond to many materials. The mold maybe constructed of material which adsorbs gases but which can be readilydegassed as by heating and/or subjecting the material to a high vacuum.Glass; steel sprayed with refractories such as MgO-Al O ZrO Cr O andZrSiO Pyroceram brand ceramic; steel coated with porcelain enamels whichare not significantly attacked by molten aluminum; and pyrolyticgraphites have been found to be suitable for making molds. Oxidizedsteel, steel sprayed with A1 and conventional graphite tend to liberateoxidizing gases and may be used as mold material only if they areproperly degassed. Stainless steel and freshly abraded steel do notcause oxidation of molten aluminum but are unsuitable as mold materialsbecause they tend to alloy with molten aluminum and, consequently, thesintered article becomes firmly bonded to the mold.

For the purposes of the specification and claims, the term non-bondingis used in connection with the tendency, or the lack thereof, of moltenaluminum metal to bond to the mold walls.

Of course, if it is desired to form a sintered metal section or elementadhering to a surface of an article as an integral part thereof, e.g., afilter formed within a tubular article, the mold selected is, at leastin part, the surface or surfaces of the article itself, and the articleselected is formed of, or coated with, a material or metal or alloy towhich the particulate aluminum metal is capable of alloying or otherwisebonding during the sintering step referred to hereinabove. Thus, afilter permanently bonded to a surrounding tubular matrix may beprepared by the present method.

The sintering atmosphere must be carefully controlled to avoid excessiveoxidation of the particulate aluminum metal because the metal ismaintained at a high temperature for a relatively long time in thesintering process. Such gases as carbon dioxide, oxygen, water vapor andchlorine are to be avoided. While natural gas may be used with properprecautions, the gases of choice for filling the sintering chamber arehelium, argon, hydrogen, nitrogen, methane, ethane and mixtures thereof.Or the chamber may simply be evacuated to a pressure below about 10millimeters Hg and preferably below about 1 millimeter Hg. Water vaporin the sintering atmosphere is particularly deleterious and if presentshould always be removed to a level below about 0.3%, by volume.

So long as the sintering atmosphere used is substantially inert to thecharge of particulate aluminum metal, the atmosphere need not be static,but may be a flowing atmosphere if desired. Thus, if it is not feasibleto seal the container during sintering, it is sufiicient to maintain apositive pressure, inside the container, of substantially inert gas.

If it is necessary or advantageous for some other reason to carry outsintering in the presence of a somewhat reactive oxidizing gas such asair, the particulate aluminum may still be satisfactorily sintered ifthe container is preselected to provide a limited amount of atmosphere,viz., less than about 40 percent by volume headspace above the charge ofmetal being sintered. Al-2Si pellets, for example, can be satisfactorilysintered even in an oxygen atmosphere maintained at ambient atmosphericpres sure if the ratio of the volume of the pellet bed to the free spaceabove the pellets in the container is 5:3 or greater, but cannot besintered if the ratio is smaller, for example, 1:1. The amount ofoxidation which can be tolerated in the sintering step depends on howmuch oxidation has already occurred during preparation of particulatemetal and in storage.

If it is necessary to carry out the sintering in the presence of anoxidizing gas, the particulate aluminum can be sacrificially protectedby a more active metal which serves as a getter. Thus, it has been foundpossible to sinter atomized pellets of aluminum metal in either aloosely or tightly covered large container which contains magnesiumpellets in one or more separate compartments or receptacles. Care mustbe taken to avoid contact between the magnesium and aluminum pelletswhich readily fuse together at sintering temperatures.

A highly important aspect of the invention is maintaining control overthe extent of melting of the charge during sintering. Preparation of thesintered particulate aluminum metal article of the invention requiressome concurrent melting during the sintering step in order to obtain anarticle with desirably high strength properties. In preparing a porousarticle, the particulate metal in the sintering container is heated to atemperature and for a time that only partial melting will take place.Melting a portion of the pellets equal to at least 15 percent of theweight of the pellets is required to obtain good bonding between thepellets so as to form an article with good structural strength.

The degree of porosity of the sintered article is controlled by theextent to which the particulate metal is melted during sintering. In thepreparation of highly porous articles, as in the preparation of filters,the desired amount of melting is in the range of about 15 to 30 percent.If somewhat less porosity is desired, or if higher strength is required,the amount of melting may be increased to as high as about percent byweight of the pellets. More than this amount of melting is notparticularly harmful if the material employed is a homogenous alloy butthere is no particular advantage in completely melting the charge. Inthe case of the heterogeneous alloys, melting more than 70% by weight ofthe pellets is disadvantageous or deleterious because of the tendencyfor any unmelted metal to segregate as by floating or settling in onepart of the melt and also because of the tendency for compositionalchanges to take place in the molten phase during subsequentsolidification of the molten metal on cooling.

More preferably, the volume of metal melted during sintering does notgreatly exceed the volume of the void spaces between the particles ofmetal in the charge prior to sintering. Thus, the amount of metal meltedin a charge of uncompressed substantially spherical pellets should notgreatly exceed about 35 percent by weight of the charge, while theamount of metal melted in an uncompressed charge of either machinedchips or irregularly shaped pellets should not greatly exceed about 50percent by weight of the charge.

The time and temperature required for the requisite amount of partialmelting can be determined from an appropriate phase diagram, or thedetermination may be made by direct experiment. Typical temperaturesemployed are: 1120 F. for Al-ZSi; 1100 F. for Al-SCu; and 1080 F. forAl-8Mg. Sintering times of about 4 hours are suitable at these sinteringtemperatures. Adequate sintering may be carried out for shorter times atsomewhat higher temperatures. Generally, temperatures above about 1250F. are to be avoided because of the increased tendency for oxidation orexcessive melting to take place. If an oxidizing gas is present in theatmosphere within the sintering container and if melting is not carriedout quickly, oxidation may give rise to sufiicient barrier materialbetween particles to prevent adequate bridging and the sintered metalwill not be coherent, i.e., will not have suflicient structural strengthto be useful.

To make temperature control simpler, i.e., to permit rapid and adequatemelting without causing melting of the entire charge, it is oftendesirable to use an aluminum alloy having a melting range of at least 25Fahrenheit degrees and more preferably at least 50 Fahrenheit degrees.Then on maintaining a temperature within the melting range, but at leastabout 3 to 10 degrees below the upper end of the melting range, one canbe reasonably sure that not all of the metal will become molten.

The amount of melting should be readily controllable, particularly inthe preparation of a filter having a preselected degree of porosity. Toolittle melting will not provide adequate bonding while too much meltingwill not provide adequate porosity in the sintered article. It should beunderstood that the selection of an aluminum alloy with a wide meltingrange is not mandatory because it is 7 possible to sinter unalloyedaluminum with the desired degree of porosity on maintaining closecontrol of temperatures and times.

After liquid phase sintering of particulate aluminum metal according tothe invention, the temperature of the sintered article should bedecreased below the solidus temperature for a time suflicient tocompletely solidify the molten phase before the sintered article isremoved from the mold.

If a high density, high strength, substantially non-porous article isdesired, the present method is readily adapted to the purpose by theinclusion of a pressing step in which the particulate material iscompressed prior to the sintering step. Such pressing may be carried outat temperatures classified as cold or warm.

Cold pressing is generally carried out at room temperature andespecially at a temperature below about 100 F.

Warm pressing is carried out at temperatures in the range of about 300to 500 F. In compressing the particulate metal at temperatures belowabout 500 F., it is usually desirable to employ particulate materialhaving irregular shapes as contrasted to spherical particles since thespherical particles are diificultly compressed to cause stickingtogether. Achievement of a specific gravity of at least 80% of thetheoretical value for aluminum is desirable.

Since the preferred method of preparing particulate aluminum materialaccording to the invention is by atomizing the alloy, when irregularlyshaped particles are de sired for cold or Warm pressing, it is necessaryto carry out the atomizing process in such a manner as to provideparticles of irregular shape. This is accomplished by atomizing thealuminum metal with a gas jet, in the case of jet atomizing, or in anenvironment, in the case of disc-atomizing, wherein the gas jet orenvironment used contains sufiicient oxygen to cause the pellets to havean irregular shape. It is still highly desirable that the atomizing gasor environment is dry, i.e., contains less than about 0.3 percent byvolume water vapor.

Sintering may be omitted altogether upon using a hot pressing operation.In carrying out hot pressing, the particulate material is pressed underthe requisite conditions of temperature and compression pressure toobtain a substantially fully densified article during the hot pressingstep. During such hot pressing, a fully bonded integral article isobtained and a subsequent step of sintering is not employed. Such hotpressing is carried out by pressing a heated charge of the particulatealuminum metal in a press, e.g., in the preheated container of anextrusion press having the die opening blanked oif by a cover plate andpreferably under an inert atmosphere. Sufiicient pressure is applied tobond the particles together and to substantially eliminate the voidspaces. Temperatures of the order of 900 to 1000 F. are generallysuitable, although some of the softer alloys may be hot pressed at atemperature as low as 800 F. Temperatures at which the alloy normallymelts are to be avoided in hot pressing. Suflicient pressure is employedin hot pressing to bring the specific gravity of the charge to at leastabout 2.43 (90% of theoretical) and more preferably to at least 2.6grams per cubic centimeter.

Because of the tendency of aluminum metal to bond to other metals atelevated temperatures, it is generally desirable in preparing the morefully densified article to subject the metal charge first to a cold orwarm pressing operation in which most of the desired increase in densityis achieved. By so doing, most of the movement of the charge within thecontainer takes place at a temperature where lubricity is no problem.The partly compressed material is then subjected to a hot pressingoperation in which the desired densification is completed underconditions at which the charge and the ram of the press are subjected toa small amount of movement so that lubricants have a chance to operatesatisfactorily at the more elevated temperature.

Generally, in carrying out both hot and cold pressing,

the specific gravity of the charge of particulate metal is brought to atleast about 2.16 in the cold pressing step,

and the further densification is carried out by hot pressing, usually atabout 900 F.

One of the big advantages of any pressing operation according to thepresent invention is that the densified material is obtained eitherdirectly in pressing or subsequently on sintering and since anysubstantial reduction in cross-section, which is a necessary consequenceof an extrusion step, is avoided, it is easier to make large articlesaccording to the present method.

Particulate material employed in making a high density article must beprotected from oxidation as indicated hereinabove during the preparationof the particulate metal, during any intermediate storage and certainlyduring sintering following cold pressing or in a hot pressing operationafter cold pressing. Hot pressing can be carried out in air but in thecase of the more active alloys, it is preferred to provide a protectiveatomsphere during the pressing operation and more critically whileheating the charge before the compression step.

The sintered or hot pressed article of the invention may be worked, ifdesired, to obtain a product having the advantages of pellet metallurgy.Methods such as rolling, extruding, forging, impact extrusion, sizingand coining may be employed. Generally, in making a worked product, thehigher density metal is used to avoid cracking, especially if thematerial is to be rolled or forged. A surprising property of thesintered or hot pressed article, however, is the ease with which voidstherein are removed on working the article.

On sintering or hot pressing a particulate heterogeneous aluminum alloyaccording to the invention, a rigid article is obtained which consistsof an aluminum or aluminum alloy matrix having very intimately, finelyand uniformly dispersed therethrough, a volumetrically coextensiveinsoluble phase consisting of an insoluble constitutent or an insolublealuminum intermetallic compound. The matrix phase comprises at least 20percent by weight of the rigid article and consists of aluminum oraluminum base-alloy as defined hereinabove.

Such articles tend to be brittle and very hard, and being resistant towear and having a high compression yield strength modulus, are desirablyused for hearing inserts.

For the purposes of the specification and claims, the term aluminummetal is to be understood to include the heterogenous alloys describedhereinabove.

An important and surprising aspect of the present invention is thefurther discovery that good sintering may still be obtained if carefulcontrol is exercised to avoid oxidation of the aluminum metal in atleast one of the two steps (1) making particulate aluminum metal and (2)sintering, where the particulate aluminum metal is sintered within 24hours after its preparation.

Good sintering may also be obtained if careful con- 60 trol is exercisedto avoid oxidation of the aluminum metal in at least two of the threesteps of (1) making particulate aluminum metal, (2) storing theparticulate aluminum metal, and (3) sintering the particulate aluminummetal. The carfeul control to avoid oxidation in the 65 several steps isalready described hereinabove.

It is especially important to avoid oxidation by contact of the aluminummetal with water or water vapor. Oxidation by reaction with H O is moredeleterious to good sintering than an equivalent amount of oxidation byreaction with oxygen or air.

The minimum requirements of care, to avoid too much oxidation at theseveral stages or steps of the present method, which still permit goodsintering when the op- 75 eration falls below this standard of care inonly one step are illustrated as follows, it being understood thatequivalent conditions may also be employed:

Jet atomizing: A stream of molten aluminum metal is broken into fineparticles by hitting it with a jet of natural gas which contains lessthan 500 parts per million (by weight) oxygen and less than 0.3 percentby volume water vapor.

Storage: The particles of aluminum are stored in a closed container atan ambient room temperature below 100 F. and at a relative humidity lessthan about 90%.

Sintering: The particulate aluminum metal is placed in a sealed glasscontainer which is either filled with an inert gas such as argon orevacuated. The container and contents are heated for 4 hours at atemperature which will bring about melting of 15 to 70 percent of themetal.

The following examples of the invention are illustrative thereof and theinvention is not to be considered limited thereto.

Example 1 A melt of aluminum alloy consisting of about weight percent ofcopper and the balance commercial aluminum was prepared by melting therequisite proportions of aluminum and copper together in a gas-firedsetting. The molten alloy was fed through conventional jet atomizinglets were collected in a dry atmosphere while a separate additional partof the pellets were chilled by collection in water. The pelletscollected in water were separated from the water within less than anhour and the pellets were 5 then dried, carefully washed with a volatilechlorinated hydrocarbon and dried.

In each case, the atomized pellets were stored in a closed container atambient room temperature and under atmospheric conditions untilsintering was attempted.

Sintering in each case was attempted by sealing the charge of pellets ina glass container having less than 40 percent by volume headspace, theheadspace being evacuated or filled with an inert gas, usually argon.The sealed container was heated for 4 hours at a temperature calculatedto produce 15 to 30 percent melting of the charge.

Whether or not adequate sintering occurred to produce a strong, rigidporous article was determined by visual and microscopic examination ofthe sintered article. Sintering is considered adequate if the pelletsare bonded to each other sufficiently that the outer layer of thearticle cannot be rubbed off by hand. Another practical test is toattempt to break, by hand, a A-inch diameter rod shaped test bar about 3to 4 inches long. The article is strong enough if it cannot be broken ontrying to bend it by hand. 25 The conditions of atomizing the metal andcollecting the pellets and the results of the sintering tests are listedin the following Table I.

TABLE I Impurities Added Collection Test No. Atomizing Gas Sinter- Vol.I-'.p.m. (\vt.) able percent Oxygen Dry 1n H 0 1 Natural Gas 0.028 100 XYes 2 do 0.44 400 X Yes. 3. do 0. 44 400 X Yes 4.... 0.44 2,000 X Yes 50.44 2, 000 X YeS 6- 0. 7 5, 000-10, 000 X N0. 7 O. 7 5, 000-10, 000 X No. 8. 0. 14 2 X Yes 9 0. 44 X Yes. lO 0.44 X No. 11 0. 8 X N 0. 12 do0.8 X No.

1 Tests numbered 6, 7, 10, 11 and 12 are comparison tests. 2 None added.

equipment and pellets of the aluminum alloy were formed in a dry,natural gas atmosphere in an enclosed container. The pellets of aluminumalloy were then stored in a dry atmosphere for about ten days beforegrams of pellets were placed in a glass tube 10 millimeters in diameterand sealed at one end. The glass tube containing the metal charge wasevacuated, flushed with argon and evacuated, before the glass tube wassealed. The glass-enclosed charge was then placed in a heating oven andheated at 110=0 F. for 4 hours. After the charge had cooled, it wasremoved from the glass tube. The loose, particulate aluminum metal hadbeen converted to a rigid, porous cylindrical article displayingsubstantial mechanical strength properties (about 1500 to 7000 poundsper square inch tensile yield strength), typical of a well sinteredarticle of aluminum-base alloy.

Example 2 A series of experiments was carried out illustrating theleeway in control of oxidation in one step (here, atomizing), so long asthe mini-mum conditions or better are met in the other two steps of theprocess. Aluminum-silicon binary alloy containing about 2 percent byweight of silicon, was melted and jet atomized with a stream of naturalgas, argon or air. In the several runs, various concentrations of watervapor and/ or oxygen were added to the atomizing gas. In most atomizingruns, part of the pel- Examination of the results shows that substantialamounts of water vapor and oxygen may be tolerated in 5 the atomizingstep if at least minimum conditions are maintained during storage .(ifany) and sintering.

In a similar series of experiments, in which there was employed thealuminum-magnesium binary alloy containing about 8 percent by weight ofmagnesium, the sensitivity of this aluminum-magnesium alloy to atomizingconditions was found to be substantially the same as for the abovealuminum-silicon binary alloy.

Example 3 A series of experiments was carried out illustrating theleeway in control of oxidation in one step (here, storage), so long asthe minimum conditions or better are met in the other two steps of theprocess. Aluminum-silicon binary alloy containing about 2 percent byweight of silicon, was melted and jet atomized with a stream of naturalgas containing less than 400 ppm. oxygen and less than 0.15 percent byvolume water vapor. The pellets were collected in a dry atmosphere. Thepellets were divided into several 7 portions which were stored undervarious conditions and lengths of time before sintering was attempted.The conditions employed in the sintering attempts were those de scribedin Example 2.

A portion of pellets stored in an open-top container for 7 20 days in aroom in which dry flowing air was maintained at 95 F. was sinterable.Another portion of pellets which was stored in an open-top container for3 days in a room maintained at 100 F. and 100% relative humidity wasstill sinterable. However, a portion of pellets stored in an open-topcontainer for 7 days under conditions of 100 F and 100% relativehumidity was not sinterable.

In additional experiments, the aluminum-silicon binary alloy containing2 percent by weight of silicon was jet atomized in a stream of aircontaining 0.14 volume percent water vapor. One portion of the pelletswas stored for 20 days in an open container in a room in which dryflowing air was maintained at 95 F. Another portion was stored 24 hoursin an open container in a room maintained at 100 F. and 100% relativehumidity.

Sintering of each portion was attempted under the sintering conditionsdescribed in Example 2. The pellets stored in dry flowing air weresinterable, showing these conditions for this length of time to be theequivalent of storage in a closed container.

The pellets stored at 100 F. and 100% relative humidity for one day werenot sinterable, whereas other pellets of the same alloy atomized by ajet of dry natural gas and stored at 100 F. and 100% relative humidityfor 3 days were sinterable.

Similar tests carried out with the aluminum-copper binary alloycontaining about 5 percent by weight of copper yielded substantially thesame results.

Example 4 A series of experiments was carried out illustrating theleeway in control of oxidation in one step (here, sintering) so long asthe minimum conditions or better are met in the other two steps of theprocess. Aluminumsilicon binary alloy containing about 2 percent byweight of silicon was melted and jet atomized with either a stream ofnatural gas or of substantially dry air. The so-obtained pellets werestored in a closed container at ambient room temperature and atmosphericconditions until sintering attempts were made.

Portions of the atomized pellets were placed individually in respectiveglass containers and the containers sealed. In the several experiments,the headspace volume was varied, and differing gases were employed inthe headspace. The approximate pellet bed volume was determined aftertapping the container lightly and the volume measured included the voidsbetween the pellets. The gas in the headspace, the relative headspacevolume and the sinterability are listed for each experiment in Table IIas follows:

TABLE II Test Ratlo, Pellet Slnter- No. Gas In The Container Bed V01. toable Headspace 13. None (Vacuum) 1L... Argon 15..-- Argon plus 3% byvolume H O vapor" 211 Gas 1 Test number 22 is a comparison test.

The method of the invention having been thus described, modificationsthereof Will at once be apparent to those skilled in the art, and thescope of the invention is to be considered limited only by thehereinafter appended claims.

We claim: I

1. The method of making a rigid aluminum metal article which comprises:heating a charge consisting es sentially of loose, particulate aluminummetal in a mold in a closed gas-imprevious container to a temperatureand for a time at which from about 15 to 70 percent by weight of thecharge becomes molten, the particles of said loose particulate aluminummetal being substantially free of oxide films thicker than about 700Angstroms,

2. The method as in claim 1 in which the temperature and time arepre-selected whereby a predetermined extent of melting is obtained and aporous article is formed.

3. The method as in claim 1 in which the mold employed is formed ofmaterial which bonds to the aluminum metal during said heating.

4. The method as in claim 1 in which the mold employed has an innersurface formed of non-bonding material with respect to aluminum metal,and said inner surface is a gas-free surface.

5. The method as in claim 1 in which a sealed glass container is usedwhich serves as both the said mold and the said closed container.

6. The method as in claim 1 wherein the loose particulate aluminum metalcomprises at least one aluminumbase alloy having a melting range of atleast 25 Fahrenheit degrees.

7. The method as in claim 1 in which the loose particulate aluminummetal comprises a heterogeneous alloy, said heterogeneous alloyconsisting of at least 20 percent by weight of homogeneous aluminumalloy and the balance a metal phase which is insoluble at normal castingtemperatures, said insoluble metal phase being completely miscible withsaid homogeneous aluminum alloy at an elevated temperature above thenormal casting temperatures.

8. The method as in claim 1 in which said loose particulate aluminummetal (I) is prepared in particulate form in a substantiallynOn-OXidiZing atmosphere, (2) is thereafter stored substantially freefrom contact with active oxidizing gases and (3) is substantiallyprotected from oxidation during subsequent sintering.

9. The method as in claim 8 in which each of the steps of preparing inparticlulate form, storing and sintering of the particulate aluminummetal is carried out in an atmosphere containing less than 0.3 percentby volume of water vapor.

10. The method as in claim 1 in which protection from contact withactive oxidizing gases is carried out in only two of the three steps ofpreparing the loose particulate metal, storing such particulate metaland heating the said charge thereof.

11. The method as in claim 1 in which the charge of loose particulatealuminum metal is subjected to a compacting pressure at a temperaturebelow about 500 F. prior to said heating.

12. The method as in claim 1 in which the atmosphere in the container isselected from the group consisting of nitrogen, hydrogen, helium, argon,natural gas, methane,

D ethane and mixtures thereof, any of the foregoing gases in addition tooxygen or air in an amount which does not occupy more than 40 percent byvolume head space above the charge in the container, and any of theforegoing gases at a pressure below about 10 mm. of mercury pressure.

13. The method as in claim 1 in which the said particulate metal isheated in the presence of a charge of loose particulate magnesium in thesaid container, both said particulate metals being exposed to the sameatmosphere.

(References on following page) 3,359,095 13 14 References Cited FOREIGNPATENTS UNITED STATES PATENTS 544,833 4/1942 Great Britain.

6/1942 J 75 200 626,764 7/1949 Great Britain.

ones

2/1966 Storchheim 75 222 5 OTHER REFERENCES 8/ 1942 Morris et a1 75 214Matenals and Methods, April 1955, pp. 98-101. 12/1945 St 7'226 X 6/1957i s 71426 X CARL D. QUARFORTH, Primary Examiner. 10/ 1957 Ennor et a175226 L. DEWAYNE RUTLEDGE, Examiner,

8/1963 Hancock et 211.

R. L. GRUDZIECKI, Assistant Examiner.

1. THE METHOD OF MAKING A RIGID ALUMINUM METAL ARTICLE WHICH COMPRISES:HEATING A CHRAGE CONSISTING ESSENTAILLY OF LOOSE, PARTICULATE ALUMINUMMETAL IN A MOLD IN A CLOSED GAS-IMPREVIOUS CONTAINER TO A TEMPERATUREAND FOR A TIME AT WHICH FROM ABOUT 15 TO 70 PERCENT BY WEIGHT OF THECHARGE BECOMES MOLTEN, THE PARTICLES OF SAID LOOSE PARTICULATE ALUMINUMMETAL BEING SUBSTANTIALLY FREE OF OXIDE FILMS THICKER THAN ABOUT 700ANGSTROMS.